A movable member (e.g., a sliding table) in machine tool system is generally driven by an actuator, such as a servomotor. However, before any motion is actually imparted to the member by the motor, a number of forces and factors must be overcome. For example, a coulomb (sliding) frictional force or torque is associated with the movable member and opposes the torque created by the revolution of the motor. Until the force generated by the motor becomes large enough to overcome this frictional force, no motion of the movable member will be obtained. Accordingly, movement of the motor will not necessarily result in movement of the member due to the coulomb friction. More specifically, if adequate compensation is not provided, the motor is unable to instantaneously respond to the torque discontinuity which occurs during reversal of the direction of travel due to the step change in the Coulomb friction force (i.e., the reversal of the direction of the force). While the motor will eventually "catch up" and reach the correct position, there is a brief period of time wherein the motor is not in the correct position.
Another example of the factors affecting machine performance is the machine characteristic known as "backlash" which is generally identified by the minuscule spaces between various parts or elements of the machine. Backlash errors are most easily observed when the machine reverses direction, because, during reversal, the machine elements move through these small spaces and are not in complete contact. Accordingly, no motion of the movable member is obtained, although motion is expected because the actuator has moved a particular distance. Thus, backlash can cause "lost motion." Lost motion, as used herein, refers to movement of the actuator which does not produce any movement of the controlled element. If uncompensated, lost motion results in a difference between the desired position of the movable member (also referred to herein as the "axis") and the actual position which is achieved.
As can be understood, these phenomena can cause errors in the performance of the machine, as well as undesirable results, such as poorly machined parts. Accordingly, to prevent such errors, compensation or correction methods have been developed in which a compensation or feedforward value is added (at appropriate times) to the position command or torque command delivered to the motor by the control. A method for compensating for the sudden changes in frictional forces, such as those which occur at axis reversal, is described in U.S. Pat. No. 5,170,498, the entire disclosure of which is hereby incorporated herein by reference. A method for backlash compensation is described in U.S. Pat. No. 3,794,902.
Another method for compensating for various factors affecting performance is known as bidirectional error compensation, which compensates for backlash as well as the error due to variation in the pitch of the ballscrew. In developing the proper levels of compensation, the member is moved incremental amounts, and the difference between the commanded position and the actual position is measured for each increment, such as by using a laser measurement system, after the member has stopped. These errors are clue to backlash as well as to variations in the pitch of the ballscrew. The rated ballscrew pitch is the measure of the expected linear movement per revolution. However, clue to imperfections in the manufacture of the ballscrew, the actual pitch may vary from the rated pitch, and this variation contributes to the error detected by the laser. The amount of error which is measured at each increment can be stored and used as a compensation value to be added to the position command whenever the machine is commanded to move to that particular location. The errors and incremental positions can be stored in a data table, known as a bi-directional error compensation table.
However, while friction compensation, backlash compensation, and bi-directional compensation methods can improve the performance of the machine, it has been found that these compensations do not completely account for all lost motion. In particular, it has been found that these compensation methods do not correct lost motion errors which exist only when the axis is in motion. It is believed that this remaining lost motion is due to a phenomena which we will refer to herein as "windup." "Windup", as used herein, refers to the compression, stretching, compliance or deflection which occurs in various machine components which connect the actuator with the movable member, such as the bearings, couplings, ball screws, and the like. For example, if the actuator attempted to push the movable member to the desired position from a resting position, these components would typically first compress a particular amount before any motion is realized. This compression results in lost motion and the errors that are associated therewith. While known compensation techniques may compensate for the type of lost motion which remains when motion has fully stopped (i.e. persistent lost motion) and which may be partially caused by windup, the type of lost motion which is present only during motion and which slowly corrects itself upon stopping remains uncompensated, and, accordingly, causes errors during motion of the axis.
Accordingly, there is a need for a method and apparatus for effectively compensating for lost motion and errors which are experienced during continuous motion of a positioning system, such as a machine tool system, and which are believed to be due to "windup."